Solder joint flip chip interconnection having relief structure

ABSTRACT

A flip chip interconnect has a tapering interconnect structure, and the area of contact of the interconnect structure with the site on the substrate metallization is less than the area of contact of the interconnect structure with the die pad. A solder mask has an opening over the interconnect site, and the solder mask makes contact with the interconnect structure, or is in close proximity to the interconnect structure, at the margin of the opening. The flip chip interconnect is provided with an underfill. During the underfill process, the contact (or near proximity) of the solder mask with the interconnect structure interferes with flow of the underfill material toward the substrate adjacent the site, resulting in formation of a void left unfilled by the underfill, adjacent the contact of the interconnect structure with the site on the substrate metallization. The void can help provide relief from strain induced by changes in temperature of the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application No.60/597,648, filed Dec. 14, 2005, titled “Semiconductor system withsolder joint interconnect”, which is hereby incorporated by referenceherein.

This application is a Continuation-in-Part of U.S. application Ser. No.10/985,654 by Rajendra D. Pendse, titled “Bump-on-lead flip chipinterconnection”, which was filed Nov. 10, 2004 now U.S. Pat. No.7,368,817; and this application is a Continuation-in-Part of copendingU.S. application Ser. No. 11/388,755 by Rajendra D. Pendse, titled “Flipchip interconnection having narrow interconnection sites on thesubstrate”, which was filed Mar. 24, 2006, both of which applicationsare hereby incorporated by reference herein.

This application is related to U.S. application Ser. No. 11/640,468,titled “Solder joint flip chip interconnection”, which is being filed onthe same date as this application, and which is hereby incorporated byreference herein.

BACKGROUND

This invention relates to semiconductor packaging and, particularly, toflip chip interconnection.

Flip chip packages include a semiconductor die mounted onto a packagesubstrate with the active side of the die facing the substrate. Thesubstrate is made up of a dielectric layer and at least one metal layer,patterned to provide substrate circuitry, which includes among otherfeatures traces (“leads”) leading to interconnect pads. The metal layermay be patterned by, for example, a mask-and etch process.Conventionally, interconnection of the circuitry in the die withcircuitry in the substrate is made by way of bumps which are attached toan array of interconnect pads on the die, and bonded to a corresponding(complementary) array of interconnect pads (often referred to as“capture pads”) on the substrate. The capture pads are typically muchwider than the leads, and can be as wide as, for example, about 2 to 4times the nominal or design width of the leads. Conventionally, theinterconnect area on the capture pad is approximately equal to theinterconnect area on the die pad.

The areal density of electronic features on integrated circuits hasincreased enormously, and chips having a greater density of circuitfeatures also may have a greater density of sites (“die pads”) forinterconnection with the circuitry on a package substrate.

The package is connected to underlying circuitry, such as a printedcircuit board (e.g., a “motherboard”), in the device in which thepackage is employed, by way of second level interconnects (e.g., pins,secondary interconnect solder balls) between the package and theunderlying circuit. The second level interconnects have a greater pitchthan the flip chip interconnects, and so the routing on the substrateconventionally “fans out”. Significant technological advances inpatterning the metal layer on the substrate have enabled construction offine lines and spaces; but in the conventional arrangement space betweenadjacent pads limits the number of traces than can escape from the moreinward capture pads in the array, and the fan out routing between thecapture pads beneath the die and the external pins of the package isconventionally formed on multiple metal layers within the packagesubstrate. For a complex interconnect array, substrates having multiplelayers may be required to achieve routing between the die pads and thesecond level interconnects on the package.

Multiple layer substrates are expensive, and in conventional flip chipconstructs the substrate alone typically accounts for more than half thepackage cost (about 60% in some typical instances). The high cost ofmultilayer substrates has been a factor in limiting proliferation offlip chip technology in mainstream products.

In conventional flip chip constructs the escape routing patterntypically introduces additional electrical parasitics, because therouting includes short runs of unshielded wiring and vias between wiringlayers in the signal transmission path. Electrical parasitics cansignificantly limit package performance.

Flip chip interconnection is commonly used in a wide variety ofintegrated circuit designs, including for example ASIC, GPU, Chipset,DSP, FPGA.

SUMMARY

The aforementioned challenges presented by conventional flip chipinterconnect can be addressed by connecting the interconnect bumpdirectly onto a lead (“Bump-on-Lead” interconnect, or “BoL”); or byconnecting the interconnect bump directly onto a narrow interconnectionpad, or narrow pad (“Bump-on-Narrow Pad”, or “BoNP”) rather than onto aconventional capture pad. Such approaches are described in, for example,related U.S. application Ser. No. 10/985,654 (BoL) and U.S. applicationSer. No. 11/388,755 (BoNP), referenced above.

BoNP or BoL approaches can provide more efficient routing of traces onthe substrate. Particularly, the signal routing can be formed entirelyin a single metal layer of the substrate. This can reduce the number oflayers in the substrate, and forming the signal traces in a single layeralso permits relaxation of some of the via, line and space design rulesthat the substrate must meet. This simplification of the substrategreatly reduces the overall cost of the flip chip package. Thebump-on-lead architecture also helps eliminate such features as vias and“stubs” from the substrate design, and enables a microstrip controlledimpedance electrical environment for signal transmission, therebygreatly improving performance.

We have discovered that, within selected design parameters, a BoL orBoNP flip chip interconnection can be at least as reliable as aconventional bond on capture pad interconnect. Generally, within arange, interconnect structures formed on narrower leads—that is, moresharply tapered interconnect structures—can be more reliable thaninterconnect structures formed on less narrow leads.

Particularly, where the CTE of the die differs significantly from theCTE of the substrate, improved reliability can result from configuringthe interconnect so that the interconnect structure is tapered, and thearea of contact of the solder with the site on the lead is significantlyless than the area of contact of the solder with the die pad. Thecontact at the site on the lead can be narrower as a result of a narrowdimension of the lead at the site (BoL) or of a narrow pad at the site(BoNP); or the contact at the site on the lead can be narrow as aconsequence of masking a larger pad or otherwise limiting thesolder-wettable area of a larger pad.

In some embodiments the CTE of the substrate is significantly greaterthan the CTE of the die; and the material of the interconnect structureis selected to be close to that of the substrate. For example, the CTEof a laminate (organic) substrate is typically in a range about 16-18ppm/degree C.; the CTE of silicon is about 2-3 ppm/degree C.; the CTE of“glass ceramic” (in earlier use for substrates) is about 3-4 ppm/degreeC.; the CTE of a co-fired ceramic (in use in multilayer substrates—asany as 16-18 layers, for example) is about 8-8.5 ppm/degree C. The CTEof a laminate (organic) substrate, on the other hands, is typically in arange about 16-18 ppm/degree C., and so a significant CTE mismatchexists between silicon die and organic laminate or build-up substrates.In some embodiments, the die is silicon-based, the substrate is anorganic laminate or build-up substrate, and the tapered interconnectstructure has a CTE in the range about 18-28 ppm/degree C.

In one general aspect the invention features a flip chip interconnectionhaving tapered interconnect structures, in which a width of theconnection of the interconnect structure with a die pad is greater thana width of the connection of the interconnect structure with a site on alead. In some embodiments the connection at the die pad is about 1.5times as wide as the connection at the lead, or is about 2 times as wideas the connection at the lead, or is about 3 times as wide as theconnection at the lead, or is about 4 times as wide as the connection atthe lead. The width of the connection at the die pad can be in a rangeabout 50 um to about 150 um; die pads in common use have widths about110 (or 120) urn and about 90 um. The width of the connection at thesite at the lead can be in a range about 20 um to about 100 um; somestandard leads have widths at the site about 50 um, or about 40 um, orabout 30 um. Where the CTE mismatch between the die and the substrate isgreater, a more sharply tapering interconnect structure may prove morereliable; where the CTE mismatch is less, a less sharply taperedinterconnect structure may prove suitable.

In some embodiments the interconnect structure is a composite structure,including a higher-melting bump connected to the die pad, and alower-melting solder connecting the bump to the site on the lead. Thelower-melting component of the composite structure can be provided as acap on the bump; or, the lower-melting component can be provided on theinterconnect site (for example as a solder paste, or a plated spot); ora lower-melting material could be provided on each the bump and thesite. The higher-melting bump can be of a material that is substantiallynon-collapsible at the reflow temperatures employed in making theinterconnect. In such embodiments the higher-melting bump can be, forexample, a high-lead solder (such as a lead-tin alloy having high leadcontent), or copper, or gold, or nickel, or a combination of these. Thelower-melting solder can be, for example, a eutectic solder, which maybe tin-based, including tin and alloys of tin such as silver, copper, orlead, or a combination of these. Or, the bump could be entirely of amaterial that melts at the reflow temperature.

The bump can be affixed to the die pad; or, it can be formed on the diepad in situ, by printing or plating the bump material at the die padsand then heating to form the bumps.

In another general aspect the invention features a flip chip packageincluding a die having interconnect pads in an active surface, and asubstrate having interconnect sites on electrically conductive traces ina die attach surface, in which tapered interconnect structures connectthe die pads to the sites. In some embodiments the sites includelocations in the leads (BoL); in some embodiments the sites includenarrow pads in the leads (BoNP); in some embodiments the sites includesmall-area portions of capture pads.

The bump-on-lead interconnection is formed according to methods of theinvention with use of a solder mask to confine the molten solder duringa re-melt stage in the process. According to the invention the soldermask and openings in the solder mask are configured so that the soldermask contacts—or is very close to—the interconnect structure at themargin of the solder mask opening, so that it interferes with the flowof underfill material to the region adjacent the connection of theinterconnect structure with the metallization on the substrate. As aresult, voids remain following completion of the underfil process.According to the invention, these voids can provide some relief fromthermal stress. The voids may contain whatever atmosphere was presentduring the underfill process.

Accordingly, in some embodiments the substrate is further provided witha solder mask having openings over the interconnect sites on the leads,and the solder mask contacts the interconnect structures at the marginof the solder mask openings; the assembly includes an underfill betweenthe die and the substrate, but voids remain adjacent the interconnectionwith the sites on the substrate, defined in part by the locus of thecontact of the solder mask with the interconnect structures.

Accordingly in one general aspect the invention features a flip chipinterconnection having tapered interconnect structures, in which a widthof the connection of the interconnect structure with a die pad isgreater than a width of the connection of the interconnect structurewith a site on a lead, and further including a solder mask havingopenings, in which the solder mask contacts the interconnect structureat the margin of the openings, and further comprising an underfillbetween the die and the substrate, wherein voids are present adjacentthe sites on the substrate.

We have also found by computer modeling that thermally-induced maximumstress in BoL interconnections is less on leads that are oriented at theinterconnect site toward a thermally neutral point on the die (that is,in a “radial” direction) than on leads that are oriented at theinterconnect site perpendicularly to a radial direction.

In another general aspect the invention features a substrate for BoL orBoNP flip chip interconnection, in which the lengthwise dimension of theinterconnect site (the mating portion of the lead or narrow pad) isoriented in a direction approximately aligned toward the thermallyneutral point of the die, or deviating less than about 45° (moreusually, less than about 20°; still more usually, less than about 10°)from such an alignment.

In another general aspect the invention features a method for formingflip chip interconnection, by providing a substrate having interconnectsites in conductive traces formed in a die attach surface, providing adie having bumps attached to interconnect pads in an active surface;providing a fusible conductive material on the bumps or on theinterconnect sites (or on each the bumps and the interconnect sites);supporting the substrate and the die; positioning the die with theactive side of the die toward the die attach surface of the substrate,and aligning the die and substrate and moving one toward the other sothat the bumps contact the corresponding sites; and melting and thenre-solidifying the fusible material, forming a metallurgicalinterconnection between the bump and the trace. In some embodiments themethod further includes forming an underfill between the die and thesubstrate.

In another general aspect the invention features a method for formingflip chip interconnection, by providing a substrate having traces formedin a die attach surface and having a solder mask having openings overinterconnect sites on the leads, and a die having bumps attached tointerconnect pads in an active surface; supporting the substrate and thedie; positioning the die with the active side of the die toward the dieattach surface of the substrate, and aligning the die and substrate andmoving one toward the other so that the bumps contact the correspondingtraces (leads) on the substrate; melting and then re-solidifying to formthe interconnection between the bump and the interconnect site on thetrace.

In some embodiments the solder bump includes a collapsible solderportion, and the melt and solidifying step melts the bump to form theinterconnection on the interconnect site. In some embodiments thesubstrate is further provided with a solder paste on the interconnectsite, and the step of moving the die and the substrate toward oneanother effects a contact between the bump and the solder on the site,and the melt and solidifying step melts the solder on the site to formthe interconnection.

In another general aspect the invention features a method for formingflip chip interconnection, by providing a substrate having traces formedin a die attach surface and having a solder mask having openings overinterconnect sites on the leads and having solder paste on the leads atthe interconnect sites, and a die having bumps attached to interconnectpads in an active surface; supporting the substrate and the die;positioning the die with the active side of the die toward the dieattach surface of the substrate, and aligning the die and substrate andmoving one toward the other so that the bumps contact the solder pasteon the corresponding traces (leads) on the substrate; and melting andthen re-solidifying the solder paste, forming a metallurgicalinterconnection between the bump and the trace.

In another general aspect the invention features a flip chip package,including interconnections formed as described above, and additionallyincluding forming an underfill between the die and the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagrammatic sketch of a portion of a conventionalbump-on-capture pad (“BoC”) flip chip interconnection, in a sectionalview parallel to the plane of the package substrate surface.

FIG. 1B is a diagrammatic sketch in a plan view showing a die mounted ona substrate in a flip chip manner, in which the die and the substratehave significantly different thermal expansion coefficients, and showingdimensional change of the die in relation to the substrate as a resultof change in temperature.

FIG. 2A is a diagrammatic sketch showing a portion of a flip chipinterconnection according to the invention, in a sectional viewperpendicular to the plane of the package substrate surface andgenerally transverse to the long axes of the leads.

FIG. 2B is a diagrammatic sketch showing a portion of a flip chipinterconnection according to the invention, in a sectional viewperpendicular to the plane of the package substrate surface andgenerally parallel long axes of the lead.

FIGS. 3A and 3B are diagrammatic sketches as in FIGS. 2B and 2B,respectively indicating dimensional references for various of thefeatures.

DETAILED DESCRIPTION

The invention will now be described in further detail by reference tothe drawings, which illustrate alternative embodiments of the invention.The drawings are diagrammatic, showing features of the invention andtheir relation to other features and structures, and are not made toscale. For improved clarity of presentation, in the FIGS. illustratingembodiments of the invention, elements corresponding to elements shownin other drawings are not all particularly renumbered, although they areall readily identifiable in all the FIGS.

The conventional flip chip interconnection is made by using a meltingprocess to join the bumps (conventionally, solder bumps) onto the matingsurfaces of the corresponding capture pads and, accordingly, this isknown as a “bump-on-capture pad” (“BoC”) interconnect. Two features areevident in the BOC design: first, a comparatively large capture pad isrequired to mate with the bump on the die; second, an insulatingmaterial, typically known as a “solder mask” is required to confine theflow of solder during the interconnection process. The solder maskopening may define the contour of the melted solder at the capture pad(“solder mask defined”), or the solder contour may not be defined by themask opening (“non-solder mask defined”).

The techniques for defining solder mask openings have wide toleranceranges. Consequently, for a solder mask defined bump configuration, thecapture pad must be large (typically considerably larger than the designsize for the mask opening), to ensure that the mask opening will belocated on the mating surface of the pad; and for a non-solder maskdefined bump configuration, the solder mask opening must be larger thanthe capture pad. The width of capture pads (or diameter, for circularpads) is typically about the same as the ball (or bump) diameter, andcan be as much as two to four times wider than the trace width. Thisresults in considerable loss of routing space on the top substratelayer. In particular, for example, the “escape routing pitch” is muchbigger than the finest trace pitch that the substrate technology canoffer. This means that a significant number of pads must be routed onlower substrate layers by means of short stubs and vias, often beneaththe footprint of the die, emanating from the pads in question.

FIG. 1A shows a portion of a conventional flip chip package, indiagrammatic sectional view; the partial sectional view in FIG. 1A istaken in a plane perpendicular to the package substrate surface.Referring now to FIG. 1A, a die attach surface of the package substrateincludes a patterned electrically conductive layer formed on adielectric layer 10. The metal layer is patterned to form leads andcapture pads 16. An insulating layer 11, typically termed a “soldermask”, covers the die attach surface of the substrate; the solder maskis usually constructed of a photodefinable material, and is patterned byconventional photoresist patterning techniques to have openings,indicated at 12, leaving the mating surfaces of the capture pads 16exposed. Interconnect bumps 17 attached to pads (so-called “under bumpmetallization”) 18 on the active side of the die 14 are joined to themating surfaces of corresponding capture pads 16 on the substrate toform appropriate electrical interconnection between the circuitry on thedie and the leads on the substrate. The active side of the die 14 iscovered, except at the contact surfaces of the die pads 16, with a diepassivation layer 15, which may be, for example, a polyimide layer.After the reflowed solder is cooled to establish the electricalconnection, an underfill material (not shown in these FIGS.) isintroduced into the space between the die and the substrate,mechanically stabilizing the interconnects and protecting the featuresbetween the die and the substrate.

In such a conventional flip chip interconnect arrangement, signal escapetraces in the upper metal layer of the substrate lead from theirrespective capture pads across the die edge location, and away from thedie footprint. The capture pads are typically three times greater thanthe trace width. In one example of a conventional arrangement, thecapture pads are arranged in a 210 um two-row area array pitch in asolder mask defined configuration, with one signal trace between capturepads in the marginal row, resulting in an effective escape pitch about105 um, for example. In one example of a BoL interconnect, theinterconnect sites can be arranged in a 210 um three-row area arraypitch, with two signal traces between sites in the outer row, resultingin an effective escape pitch about 70 um. This escape pitch is adequateto route a significant proportion of integrated circuit designs thatcommonly employ flip chip interconnection on a single metallizationlayer, based on the inherent I/O density of the IC device architectures.BoL interconnection also further opens the prospect of routing aconsiderable proportion of flip chip designs in conventionalthrough-hole laminate substrates, inasmuch as laminate substrates haveline/space capacities of about 40 um/40 um (or better). This couldprovide for substantial cost reduction.

FIG. 1A shows a solder mask defined solder contour. As the fusiblematerial of the bumps on the die melts, the molten solder tends to “wet”the metal of the capture pads, and the solder tends to “run out” overany contiguous metal surfaces that are not masked. The solder tends toflow along the underlying pad (and exposed contiguous lead), and in thesolder mask defined contour the solder flow is limited by the soldermask, for example by the width of the opening 12 in the solder mask 11.

A non-solder mask defined solder contour may alternatively be employed,in which the flow of solder along the lead is limited at least in portby a patterned deposition of non-solder-wettable material on the leadsurface.

Thermal movement (in the x-y plane) of die pads on the die attachsurface of the die in relation to the corresponding points on thesubstrate (as indicated for example by arrow 13 in FIG. 1A), can resultin stresses to the interconnections between the die pad and the site onthe substrate. Dimensional change, resulting from temperature changes,of a flip chip mounted die in relation to a substrate, is showndiagrammatically (and with dimensions exaggerated) in plan view in FIG.1B. In this example, there is a significant mismatch between the CTE ofthe die and the CE of the substrate. A portion of the substrate is shownat 11. Dimensional change of the die in the x-y plane (parallel to theplane of the substrate) in relation to the substrate as the temperaturechanges (for example, during thermal cycling in assembly or test or dieburnout routines) is shown by the arrows 113. A footprint of the die ata higher temperature (the substrate is more “thermally expanded” thanthe die) is shown at 14B; a footprint of the die at a lower temperatureis shown at 14B. As will be appreciated, registration of any point onthe active surface of the die 14 with respect to a correspondingunderlying point on the die attach surface of the substrate 11 willchange as the dimensions of the die and substrate change differentiallyas a result of thermal stress. At some point on the active surface ofthe die there is no net movement with relation to the correspondingpoint on the underlying substrate, as a result of expansion orcontraction of the die in relation to the substrate; that point may bereferred to herein as the “thermally neutral point”. Generally, as maybe appreciated, the thermally neutral point may approximately coincidewith the geometric center of the die surface. The extent of movement ofany point on the die in an x-y plane (parallel to the plane of thesubstrate) in relation to the substrate as a result of thermal expansionor contraction (the “thermal movement”) depends at least in part uponthe distance of that point from the thermally neutral point on the die;accordingly, there is greater relative thermal movement at points nearerthe edges of the die (and, particularly, near the corners of the die)than at points nearer the thermally neutral point.

Movement (in the x-y plane) of a die pad in relation to an underlyingcontact pad can result in stresses to the interconnection between thepad and the contact pad. Where the movement passes a limit, somethinghas to give: failure of the interconnect can result. In conventionalflip chip interconnects, where there is a thermal mismatch between thedie and the substrate, failure typically occurs at the joint between thesolder bump and the die pad. And, in convention flip chip interconnects,where there is a thermal mismatch between the die and the substrate,even if there is no failure, thermal stress at the die pad can causedamage to the die.

Conventionally it is thought that, in an ideal solder joint structure,the area (diameter) of the interconnect pad on the substrate isapproximately equal to the area (diameter) of the interconnect pad onthe die, as shown by way of example in FIG. 1A.

By analysis of interconnect configurations using computer simulations,we have determined that the primary locus of stress is as referenced at19 in FIG. 1A. Particularly, where the thermal movement of the die inrelation to the substrate is as shown at arrow 13 in FIG. 1A, thegreatest (“Maximum”) plastic strain on the interconnect is predicted bythe computer model to be at the “leading edge” 19 of the connection(interface) between the solder 17 and the die pad (UBM) 18. (As will beappreciated, the arrow might be positioned at the substrate, butreversed, showing relative movement of the substrate in the oppositedirection; the relative movement is pointed out here in relation to thedie, because the thermally neutral point is established in relation tothe die footprint. The appearance of actual failures during acceleratedfatigue testing is consistent with the model, that is, failure inconventional bond-on-capture pad interconnects usually appears at theinterface of the solder with the die pad, rather than at the interfaceof the solder with the capture pad on the substrate.

A BoL interconnection according to an embodiment of the invention isshown by way of example in sectional views perpendicular to the surfaceof the substrate in FIGS. 2A and 2B. In FIG. 2A, two solder joints areshown in a sectional view transverse to the lead, and in FIG. 2B, onesolder joint is shown in a sectional view parallel to the lead.Referring to FIG. 2A, a die attach surface of the package substrateincludes a patterned electrically conductive layer formed on adielectric layer 20. The metal layer is patterned to form leads havinginterconnect sites 26. An insulating layer 21, typically termed a“solder mask”, covers the die attach surface of the substrate; thesolder mask is usually constructed of a photodefinable material, and ispatterned by conventional photoresist patterning techniques to haveopenings, indicated at 22, leaving the top surface and the sides of thelead (the “mating surfaces”) exposed at the interconnect site 26.Interconnect structures 27 are attached to pads (so-called “under bumpmetallization”) 28 on the active side of the die 24 and are joined tothe mating surfaces of the leads at the interconnect sites 26 on thesubstrate to form appropriate electrical interconnection between thecircuitry on the die and the leads on the substrate. The active side ofthe die 24 is covered, except at the contact surfaces of the die pads26, with a die passivation layer (such as a polyimide layer) 25. Afterthe reflowed solder is cooled to establish the electrical connection, anunderfill material (not shown in these FIGS.) is introduced into thespace between the die and the substrate, mechanically stabilizing theinterconnects and protecting the features between the die and thesubstrate.

Referring now to FIG. 2B, an interconnect is shown in a sectional viewtaken along the line 2B-2B in FIG. 2A. (In this view, the solder mask 21is not shown.) This view shows the solder of interconnect structure 27covering the sides of the lead 26.

The interconnect structures according to some embodiments can be madeusing entirely fusible materials, or using composite bumps, or using asolder-on-lead method, as described above.

Particularly, for example, so-called composite interconnect structuresmay be used. Composite structures have at least two bump portions, madeof different materials, including one which is collapsible under reflowconditions, and one which is substantially non-collapsible under reflowconditions. The non-collapsible portion is attached to the interconnectpad on the die; typical conventional materials for the non-collapsibleportion include various solders having a high lead (Pd) content, forexample, (such as a lead-tin alloy having high lead content), or copper,or gold, or nickel, or a combination of these. The collapsible portionis joined to the non-collapsible portion, and it is the collapsibleportion that makes the connection with the interconnect site on thelead. Typical conventional materials for the collapsible portion of thecomposite bump include eutectic solders, for example, which may betin-based, including tin and alloys of tin such as silver, copper, orlead, or a combination of these.

This structure can be formed in the following way, for example. Solderbumps (or balls) are attached to or formed on the die pads (under bumpmetallization or UBM). Solder is applied to the interconnect sites onthe traces, for example in the form of a solder paste. The die isoriented, active side facing the mounting surface of the substrate, sothat the bumps on the die are aligned with the respective interconnectsites on the leads, and the die is moved toward the substrate to bringthe bumps into contact with the solder on the leads. The assembly isthen heated, to reflow the solder and form the connection at theinterconnect site. As the solder on the lead reflows, it wicks to thesolder-wettable surface of the solder bump, and to the solder-wettablemating surfaces of the lead. The surface of the substrate dielectric 20is not solder-wettable, and the solder tends to make little or nocontact with the substrate dielectric. The tapered form of theconnection structure (as viewed in section across the lead, as in FIG.2A) results from the narrow dimension of the lead at the interconnectsite, and the wicking of the solder during reflow.

For example, the bumps may be formed of a high-lead (high-Pb) solder(e.g., 97% lead, 2% tin), and the solder on the interconnect site can bea eutectic solder. Reflow in some such examples can be carried out at apeak temperature of 235° C., employing flux in a jet flux method.

In a BoL construct such as is shown here, although the width of theleads may vary over their length, no particular widening of the leads isformed at the interconnect sites; in a BoNP construct, the leads may bewidened to a limited extent at the interconnect sites. In either BoL orBONP construct, the sides of the lead—as well as the top—(the matingsurfaces) are exposed to the solder at the interconnect site, and duringreflow solder wicks to the solder-wettable surfaces.

As noted with reference to FIG. 2B, a solder mask is employed in theseembodiments to limit the flow of solder along the length of the leads.In other embodiments the leads may be treated to be non-solder-wettablealong portions of the leads adjacent the mating surfaces at theinterconnect sites, so that flow of solder away from the interconnectsites along the leads is limited without use of a solder mask.

Any of a variety of substrate types can be employed according to theinvention, including for example build-up film substrates and laminatesubstrates. For example, a 1-2-1 high-density build-up substrate can beused (such as an Ajinomoto Build-Up Film, or other high densitysubstrate build-up film), or a 4-layer laminate substrate can be used.

Testing of samples constructed generally as shown in FIGS. 2A and 2B andemploying a high-lead solder ball and a eutectic solder on theinterconnect site shows that eventual failure can occur at or near thelead, rather than at the die pad, as in the conventional interconnect.Moreover, in embodiments according to the invention, where thedifference between the areas of contact at the die pad and at the leadis sufficiently great, a greater number of thermal cycles to failure canbe obtained.

FIGS. 3A and 3B are similar to FIGS. 2A and 2B, marked up for referenceto dimensions of certain of the features. The features are referenced asfollows: H, interconnect height as measured from the die surface to thesolder mask surface; D, bump diameter at half the interconnect height(H/2); UD, under bump metallization diameter; OPx, mask opening width inthe x-direction (across the lead); OPy, mask opening width in they-direction (along the lead); CW, width of the (Copper) lead at theinterconnect site; CH, thickness (height) of the lead at theinterconnect site; T, solder mask thickness.

With further reference to FIG. 2A, a flip chip package according to theinvention includes an underfill 47, which contributes to the mechanicalstability of the structure and serves to protect the interconnection andthe die and substrate surfaces. As inspection of FIG. 2A shows, thesolder mask makes contact with (or is in close proximity to) theinterconnect structure 27 at the margin of the opening 22. During theunderfill process, the contact (or near proximity) of the solder maskwith the interconnect structure interferes with flow of the underfillmaterial toward the substrate adjacent the site. This results in thecreation of voids 48 left unfilled by the underfill material, adjacentthe contact of the interconnect structure with the site on the substratemetallization. Conventionally, it is generally believed that any voidsin a flip chip construct are undesirable, for a variety of reasons.However, we have discovered that these voids adjacent the interconnectcite on the substrate can help provide relief from strain induced bychanges in temperature of the system; and the generally perceivedcomplications imposed by voids can be mitigated.

A BoL construct according to the invention may have the followingdimensions, for example: UD, 90 um; D, 0.110 um; H, 75 um; T, 40 um; CW,30 um, CT, 20 um. A BoL construct having these dimensions formed on anAjinomoto Build-Up Film (ABF) 1-2-1 substrate has performed well infatigue failure tests. This result is surprising, because it isconventionally believed that preferred interconnects should have a shapeand support area for the joint at the die side approximately equal tothat on the substrate side (bond-on-capture pad, or BoC). (A similar BoLconstruct, having a wider CW (40 um) and formed on a 4-layer BTlaminate, performed less satisfactorily.) Without intending to be boundthereto, the applicants suggest that the following hypothesis mightexplain this surprising result. Although in the conventional BoC theaverage strain on the interconnect system is determined by the magnitudeof the CTE mismatch between the die and the substrate, a high strainconcentration occurs at the bump/die interface, because at this locationthere is an abrupt difference in CTE. Accordingly, applicants suggestthat fatigue failure is driven by a concentration of plastic strain atthis location, and not by the average strain. In the BoL construct,because the interconnect structure is tapered, the portion of it nearerthe substrate has a greater compliancy; particularly, applicants suggestthat there is a high compliancy region (or “relief structure”) at ornear the narrow interface at the interconnect site on the trace.Applicants suggest that this relief structure has an effect of diffusingthe strain away from the die pad, resulting in improved fatigue life ofthe system. A computer analysis (Finite Element Modeling, FET) can lendsupport to this view.

A conventional (BoC) construct having the following dimensions was usedfor the FET analysis: UD, 90 um; D, 0.110 um; H, 75 um; OPx, 95 um; T,40 um; CW, 115 um, CT, 20 um. The analysis showed a considerableconcentration of maximum strain at the interface with the die pad in theBoC model, and a maximum plastic strain in a zone on the “leading edge”of the structure at the die pad (see, FIG. 1A). In the BoL model, themaximum plastic strain is reduced, and it is shifted away from the diepad interface.

Other embodiments are within the following claims.

1. A flip chip interconnection having a tapered interconnect structure,wherein a width of the connection of the interconnect structure with adie pad is greater than a width of the connection of the interconnectstructure with a site on a lead, the interconnection further including asolder mask having openings, wherein the solder mask contacts theinterconnect structure at the margin of the openings, and furthercomprising an underfill between the die and the substrate, and voids arepresent adjacent the sites on the substrate, the ratio of the connectionat the die pad being in a range about 1.5 to 4 times as wide as theconnection at the site on the lead.
 2. The flip chip interconnection ofclaim 1 wherein the connection at the die pad is about 1.5 times as wideas the connection at the lead.
 3. The flip chip interconnection of claim1 wherein the connection at the die pad is about 2 times as wide as theconnection at the lead.
 4. The flip chip interconnection of claim 1wherein the connection at the die pad is about 3 times as wide as theconnection at the lead.
 5. The flip chip interconnection of claim 1wherein the connection at the die pad is about 4 times as wide as theconnection at the lead.
 6. The flip chip interconnection of claim 1wherein the connection at the die pad is in a range about 50 um to about150 um.
 7. The flip chip interconnection of claim 6 wherein theconnection at the die pad is about 110 um.
 8. The flip chipinterconnection of claim 6 wherein the connection at the die pad isabout 120 um.
 9. The flip chip interconnection of claim 6 wherein theconnection at the die pad is about 90 um.
 10. The flip chipinterconnection of claim 1 wherein the connection at the site on thelead has a width in a range about 20 um to about 100 um.
 11. The flipchip interconnection of claim 10 wherein the connection at the site onthe lead has a width about 50 um.
 12. The flip chip interconnection ofclaim 10 wherein the connection at the site on the lead has a widthabout 40 um.
 13. The flip chip interconnection of claim 10 wherein theconnection at the site on the lead has a width about 30 um.
 14. The flipchip interconnection structure of claim 1 wherein the interconnectstructure is a composite structure, comprising a first componentconnected to the die pad, and a second component connecting the firstcomponent to the site on the lead, the first component having a highermelting temperature than the second component.
 15. The flip chipinterconnection of claim 14 wherein the second component of thecomposite structure comprises: a bump; and a cap on the bump.
 16. Theflip chip interconnection of claim 14 wherein the second component ofthe composite structure is provided on the interconnect site.
 17. Theflip chip interconnection of claim 14 wherein the second component ofthe composite structure comprises a solder paste.
 18. The flip chipinterconnection of claim 14 wherein the second component of thecomposite structure comprises a plated spot.
 19. The flip chipinterconnection of claim 14 wherein the first component of the compositestructure comprises a material that is substantially non-collapsible ata reflow temperature employed in making the interconnect.
 20. The flipchip interconnection of claim 19 wherein the first component of thecomposite structure comprises a high-lead solder.
 21. The flip chipinterconnection of claim 19 wherein the first component of the compositestructure includes a material selected from the group consisting ofcopper, gold, and nickel.
 22. The flip chip interconnection of claim 14wherein the second component of the composite structure includeseutectic solder.
 23. The flip chip interconnection of claim 22 whereinthe eutectic solder is a tin-based solder.
 24. The flip chipinterconnection of claim 15 wherein the bump is affixed to the die pad.25. The flip chip interconnection of claim 15 wherein the bump is formedon the die pad in situ.
 26. The flip chip interconnection of claim 15wherein the bump is formed on the die pad by printing bump material atthe die pads and then heating to form the bump.
 27. The flip chipinterconnection of claim 15 wherein the bump is formed on the die pad byplating bump material at the die pads and then heating to form the bump.